Tris(trimethylsilyl) Phosphite as an Efficient Electrolyte Additive for a High-Voltage Li-Rich Disordered Rocksalt Cathode

Mn-based Li-rich cation-disordered rocksalt (DRX) materials have emerged as a promising alternative to Ni-rich layered transition-metal oxides thanks to their high energy density and their potentially lower cost. However, their operation at high voltages, up to 4.8 V vs Li/Li +, is required to activate oxygen redox and access high capacities, accelerating the electrolyte and the full system degradation. Here we address this voltage-driven instability by screening electrolyte additives in conventional carbonate-based electrolytes using the model DRX compound Li 2 MnO 2 F (LMOF). Among the tested additives, tris(trimethylsilyl) phosphite (TMSPI) stands out, improving capacity retention by 60% relative to the baseline after 80 cycles and nearly doubling the discharge rate capability at 5C (170 vs 90 mAh·g –1 ). The beneficial effect of TMSPI is further validated in LMOF//graphite full cells, showing a satisfactory capacity (220 mAh·g –1 ) and cycling stability. The mode of action of TMSPI was elucidated through a multitechnique investigation combining operando online electrochemical mass spectrometry (OEMS), electrochemical characterization, and post-mortem analyses. TMSPI enhances the electrochemical performance by mitigating manganese cation dissolution, preserving mechanical integrity of the electrode through the suppression of aluminum current collector corrosion and limiting impedance growth at the positive electrode interface. OEMS experiments further reveal that extensive gas evolution occurs concomitantly with oxygen redox activity, linking electrolyte degradation to oxygen release from the material. Although TMSPI does not suppress outgassing, it effectively mitigates the formation of acidic species through scavenging of protons, water, and fluoride ions, leading to the formation of silane derivatives such as (CH 3 ) 3 SiF, (CH 3 ) 2 SiF 2, and SiF 4 . This work demonstrates that electrolyte engineering through rational additive design offers a simple yet scalable route to significantly improve the performance of high-voltage DRX positive electrodes.